1. Field of the Invention
This invention relates to a vehicle transmission control system and more particularly to an electronic control system which is readily controllable by an operator in response to different operating conditions and which provides safe, efficient and reliable performance in each operating condition. The system of the invention is particularly advantageous for the control of transmissions of large off-road vehicles but has features usable in other applications.
2. Description of the Prior Art
The prior art contains many disclosures of electronic systems for the control of transmissions which transmit power between an engine and the vehicle traction means or other loads. For example, such systems have been the subject of several prior patents of the assignee of the present invention, including U.S. Pat. No. 4,425,620 to Batcheller et al., entitled "Electrical Control for Power Shift Transmission", and U.S. Pat. No. 4,855,913 to Brekkestran et al., entitled "Electronic Control System for Powershift Transmission", the disclosures of which are hereby incorporated by reference.
The systems of these prior patents are particularly designed for use in large off-road vehicles, such as those utilized in agricultural, construction and related applications and include a number of features which are particularly advantageous in such use. Such features include provisions for various modes of operation and the provision of a large number of forward and/or reverse gear ratios to permit an operator to obtain the ready and accurate control of movement of enormous loads without the overload of an engine and the clutches of a drive train. The operator may use an upshift/downshift control lever to request a speed change, accomplished by the system through the selection of an appropriate gear ratio, by the control of solenoid valves which hydraulically control the engagement of hydraulic clutches corresponding to an appropriate gear ratio. For example, the operator in a forward manual mode may operate the control lever to obtain a single gear ratio change to a next higher ratio, or by holding the lever may obtain sequential upshifts at predetermined time intervals, and may thereby readily obtain an optimum ratio as required by existing conditions.